1. Field of the Invention
The present invention generally relates to radiation apparatuses and systems thereof; more particularly it relates to a radiation generating apparatus including a movable diaphragm unit having a function of adjusting a radiation field passing therethrough, and to a radiation imaging system including such apparatus.
2. Description of the Related Art
In general, a radiation generating apparatus includes a movable diaphragm unit (hereinafter interchangeably referred to as “diaphragm unit” or “diaphragm”). The diaphragm unit has a function of adjusting the radiation field by blocking radiation that is unnecessary for imaging and thus reducing the exposure of a subject to radiation. The radiation field is adjusted by adjusting the size of an aperture defined by limiting blades that allows radiation to pass therethrough. Typically, the diaphragm unit has an additional function as a projector-collimator system in which the radiation field is simulated by a visible-light field so that the radiation field can be visually checked prior to imaging.
Japanese Patent Laid-Open No. 7-148159 discloses a movable X-ray diaphragm device which is used to adjust the size of an X-ray field and the size of a visible-light field to be made the same as each other. Specifically, the movable X-ray diaphragm device, by limiting with light blocking plates, makes the size of an X-ray field the same as the size of a visible light field emitted from a light source; the light source is larger than an X-ray focal point.
To adjust the size of the radiation field, the related-art diaphragm unit includes a reflector plate that is obliquely oriented. Therefore, the size of an envelope that houses the diaphragm unit becomes large, making it difficult to reduce the size of the radiation generating apparatus as a whole. The envelope is made of a material whose mass is large enough to block radiation. Therefore, as the envelope becomes larger, the mass of the envelope becomes larger too.
In such a related-art diaphragm unit employing a reflection radiation tube, a heel effect that may occur in the reflection radiation tube is advantageously reduced with the presence of the obliquely oriented reflector plate. In contrast, if a transmission radiation tube, which does not produce the heel effect, is employed, the variation in the quality of radiation worsens.